arXiv subjects

T. Sawada

Publications and source records attributed to T. Sawada.

39 records · Page 3Linked to original sources

arXiv2007

Arc-like distribution of high CO(J=3-2)/CO(J=1-0) ratio gas surrounding the central star cluster of the supergiant HII region NGC 604

We report the discovery of a high CO(J=3-2)/CO(J=1-0) ratio gas with an arc-like distribution (``high-ratio gas arc'') surrounding the central star cluster of the supergiant HII region NGC 604 in the nearby spiral galaxy M 33, based on multi-J CO observations of a 5' $\times$ 5' region of NGC 604 conducted using the ASTE 10-m and NRO 45-m telescopes. The discovered ``high-ratio gas arc'' extends to the south-east to north-west direction with a size of $\sim$ 200 pc. The western part of the high-ratio gas arc closely coincides well with the shells of the HII regions traced by H$α$ and radio continuum peaks. The CO(J=3-2)/CO(J=1-0) ratio, R_{3-2/1-0}, ranges between 0.3 and 1.2 in the observed region, and the R_{3-2/1-0} values of the high-ratio gas arc are around or higher than unity, indicating very warm (T_kin > 60 K) and dense (n(H_2) > 10^{3-4} cm^{-3}) conditions of the high-ratio gas arc. We suggest that the dense gas formation and second-generation star formation occur in the surrounding gas compressed by the stellar wind and/or supernova of the first-generation stars of NGC 604, i.e., the central star cluster of NGC 604.

astro-ph↗

arXiv2005

Gas-Solid Coexistence in Highly Charged Colloidal Suspensions

Aqueous suspensions of highly charged polystyrene particles with different volume fractions have been investigated for structural ordering and phase behavior using static light scattering (SLS) and confocal laser scanning microscope (CLSM). Under deionized conditions, suspensions of high charge density colloidal particles remained disordered whereas suspensions of relatively low charge density showed crystallization by exhibiting iridescence for the visible light. Though for unaided eye crystallized suspensions appeared homogeneous, static light scattering measurements and CLSM observations have revealed their inhomogeneous nature in the form of coexistence of voids with dense ordered regions. CLSM investigations on disordered suspensions showed their inhomogeneous nature in the form coexistence of voids with dense disordered (amorphous) regions. Our studies on highly charged colloids confirm the occurrence of gas-solid transition and are in accordance with predictions of Monte Carlo simulations using a pair-potential having a long-range attractive term [Mohanty and Tata, Journal of Colloid and Interface Science 2003, 264, 101]. Based on our experimental and simulation results we argue that the reported reentrant disordered state [Yamanaka et al Phys. Rev. Lett. 1998, 80, 5806 and Toyotama et al Langmuir, 2003, 19, 3236] in charged colloids observed at high charge densities is a gas-solid coexistence state.

cond-mat.soft↗

arXiv2004

A Molecular Face-on View of the Galactic Centre Region

We present a method to derive positions of molecular clouds along the lines of sight from a quantitative comparison between 2.6 mm CO emission lines and 18 cm OH absorption lines, and apply it to the central kiloparsecs of the Milky Way. With some simple but justifiable assumptions, we derive a face-on distribution of the CO brightness and corresponding radial velocity in the Galactic centre without any help of kinematical models. The derived face-on distribution of the gas is elongated and inclined so that the Galactic-eastern (positive longitude) side is closer to us. The gas distribution is dominated by a barlike central condensation, whose apparent size is 500x200 pc. A ridge feature is seen to stretch from one end of the central condensation, though its elongated morphology might be artificial. The velocity field shows clear signs of noncircular motion in the central condensation. The `expanding molecular ring' feature corresponds to the peripheral region surrounding the central condensation with the Galactic-eastern end being closer to us. These characteristics agree with a picture in which the kinematics of the gas in the central kiloparsec of the Galaxy is under a strong influence of a barred potential. The face-on distribution of the in situ pressure of the molecular gas is derived from the CO multiline analysis. The derived pressure is found to be highest in the central 100 pc. In this region, the gas is accumulating and is forming stars.

astro-ph↗